Investigations of Cu₂ZnSnS₄-based (CZTS) solar cells have been performed. The configuration of the solar cell sample is In/CZTS/n-GaN, in which sapphire is used as the substrate. 1/f noise (10-100 kHz) is investigated for the CZTS solar cells through spectral characterization. I-V measurement was performed to choose the proper method of measuring low frequency noise. A model, in which the carriers are trapped and detrapped by the localized states via thermal activation processes, is used to interpret the experimental data of low frequency noise. To estimate the concentration of impurities in CZTS solar cell, the sample was characterized using C-V measurements.